The conventional fabrication of a solar module (of the type in which a plurality of layers of thin film, generally amorphous silicon alloy material are sequentially deposited onto a web of substrate material so as to form a p-i-n solar cell) can be divided up into two generic processes: namely, the front-end fabrication process and the back-end fabrication process. Generally speaking, the front-end fabrication process includes such fundamental steps, relating to the deposition of the active layers of amorphous silicon alloy solar cell material as: providing a substrate, such as an elongated web of stainless steel; mechanically and/or plasma cleansing of the substrate; depositing one or more back reflector layers atop the cleansed surface of the stainless steel substrate; sequentially depositing at least one triad of layers of n-i-p type amorphous silicon alloy material and/or amorphous silicon-germanium alloy material atop the back reflector layers; and depositing one or more thin film layers of a transparent conductive oxide, as the upper electrode, and magnesium fluoride, as the anti-reflective coating, atop the layers of amorphous silicon alloy material.
The back end fabrication process includes such fundamental steps, relating to photovoltaic module fabrication, as: passivating short circuit defects which are inherently present in the deposited layers of silicon alloy and/or silicon germanium alloy material; printing a grid pattern onto the exposed surface of the upper indium tin oxide electrode; attaching a plurality of regularly spaced, individual, elongated, electrically conductive strips between adjacent rows of grid fingers; securing bus bars along the longitudinal extent of the uncoated side of the substrate; completing the electrical interconnection of small area cells of the large area panel; and cutting the substrate into a predetermined individually selected module size, such as 48 inches by 12 inches.
Finally, the photovoltaic panel requires that a layer of lightweight, durable, readily formable material be laminated onto, and encapsulate, one surface of that photovoltaic panel. The laminate may be selected from any material which is sufficiently durable to environmental seal the solar cell module so as to withstand ambient conditions. In order to bond the laminate to the first surface of the photovoltaic module, a flowable organic resin, such as a sheet of EVA (ethylvinylacetate) of a uniform thickness of about 18 mil. This flowable organic resin may be also effected by any number of methods currently known and employed by those ordinarily skilled in the art, as for instance, by spray coat deposition with any excess removed by a doctor blade or squeegee. It is to be noted that as the EVA flows, irregularities present on the surface of the photovoltaic module are filled in and an environmental shield against ambient contamination is formed about that first surface. However, the layer of EVA is soft and an outer encapsulant is still required in order to provide greater mechanical rigidity.
Accordingly, after the deposition or other disposition of the relatively thick layer of EVA, an elongated sheet of about 1 to 10 mil thick, and preferably 3-6 mil thick, dielectric organic polymeric resin of such as TEDLAR (registered trademark of Dupont Corporation), is placed atop the EVA layer. Note that this combination of thicknesses of dielectric materials (EVA and TEDLAR) must be sufficient to electrically insulate the photogenerated current carried by the photovoltaic panel from the user.
In order to complete the lamination process and fully encapsulate the photovoltaic panel, it is then necessary to deposit a layer of the flowable organic resin to the opposite, for instance, light incident, surface of the photovoltaic panel. In a preferred embodiment, a sheet of EVA is uniformly placed thereacross, said sheet having a thickness of about 10 to 30 mils and preferably about 15-25 mils. Finally, a sheet of thin, but relatively hard, abrasion resistant, durable, flexible, optically transparent, hermetically sealable material is placed or deposited upon the uppermost layer of EVA. This uppermost layer is adapted not only to protect the photovoltaic device from harsh environmental conditions, but also must be transparent to all wavelengths of the incident solar spectrum to which the photovoltaic device is capable of photogenerating charge carriers, i.e., from about 35 to 100 nanometers. In preferred embodiments of the instant invention, said relatively hard, durable layer can be TEDLAR or TEFCEL (each being a registered trademark of Dupont Corporation) of approximately 2 to 10 and preferably about 3-6 mils thickness. It should also be noted that a glass plate may be employed in place of the transparent light incident encapsulant and that a metallic, fiberglass or wood back plate can be employed in addition to the rear encapsulant if added rigidity of the photovoltaic module is required. Such rigid members can be bonded to the module utilizing the improved lamination process of the instant invention.
The last step in the encapsulating process involves the curing of the laminated stratified sandwich of upper and/or lower layers so as to provide an integrated photovoltaic module structure. Specifically, in order to effect the lamination of the outermost encapsulating sheets to the subjacent structure of the photovoltaic panel, it is necessary to employ the correct combination of pressure, temperature and time parameters.
It is to an improvement in the heretofore employed lamination/EVA curing process and associated apparatus that the instant invention is directed. More specifically, prior to the invention, conventionally employed curing apparatus were designed to urge one of a pair of spacedly disposed inflatable rubber bladders against the polymeric encapsulant/EVA/surface of the photovoltaic module and the other of the pair of inflatable bladders against the opposite surface of the module. The EVA had to be cured at a temperature of above at least about 120.degree. C. at a pressure of about one (1) atmosphere in order to cross-link the molecular bonds of the constituent molecules thereof, and laminate the hard, durable synthetic resin encapsulating layer to the photovoltaic panel. Heretofore, the lamination occurred by placing discrete panels, one at a time, adjacent the large area inflatable bladder for providing the pressure required to cure the EVA and laminate the encapsulating layer. Machines currently designed to effect such two bladder-type lamination are very costly (about $60,000), can only laminate an encapsulant onto a single substrate per cycle and must be maintained under pressure during cool-down so as to prevent warpage of the encapsulating sheet.
It is important to note that the assignee of the instant invention employs a specific embodiment of the aforedescribed photovoltaic cell deposition process wherein a one-thousand foot long, fourteen inch wide roll of stainless steel is continuously coated with successive thin film layers of amorphous silicon alloy material. Upon completion of the back-end processing, the continuous 1000 foot long roll of photovoltaic panels must be cut and encapsulated. As should be apparent to the reader hereof, the task of then encapsulating 250 of these 4 foot long panels in a process which can only be accomplished in a one-at-a-time manner is monumental. It is to the end of improving the throughput of photovoltaic panels, and hence, the economic handling capabilities of this commercial encapsulation technique that the instant invention is directed.